Sound reproducing system



July 23, 1935- J. H. HAMMOND, JR., ET-AL 2,008,706

SOUND REPRODUCING SYSTEM Original Filed Jan. 22, 1951 Q I [N VENTORS :5 M 91 M A TTORNE Y Patented July 23, 1935 UNITED STATES PATENT OFFICE Application January 22, 1931, Serial No. 510,384 Renewed September 22, 1933 11 v Claims. (Cl. 179100.1)

This invention relates to sound reproducing systems, and more particularly to an improved system for automatically expanding or multiplying the dynamic range of the sound energy.

The invention provides more specifically for the simultaneous control of the gain ratio or amplification of a plurality of cascaded stages of amplification in accordance with the energy applied to the system whereby the volume ratio of the signal may be expanded.

The invention further provides various Initial biasing potentials and difierent rates of change of bias for the diiferent stages of amplification so that each stage may operate with the greatest possible freedom from tone distortion. I

A further purpose of the invention is to allocate the dynamic multiplication by a plurality of cascaded stages to different amplifying devices in accordance with the signal strength in these stages, with less multiplication on stages with greater signal.

The invention also consists in certain new and original features of construction and combination of parts hereinafter set forth and claimed.

Although the novel features which are believed to be characteristic of this invention will be particularly pointed out in the claims appended hereto, the invention itself, as to its objects and advantages the mode of its operation and the manner of its organization may be better understood by referring to the following description taken in connection with the accompanying drawing forming a part thereof, in which:

The figure illustrates diagrammatically the system as applied to the reproduction of the sound record of a talking moving picture projector and phonograph. a

In the following description and in the claims, parts will be identified by specific names for convenience, but they are intended to be as generic in their application to similar parts as the art will permit.

Referring to the accompanying drawing, a.'

phonograph pick-up III is shown which is connected to two contacts of a double-pole doublethrow switch II, the other contacts of which are connected through a transformer I2 to a photoelectric cell l3. A lamp I4 is provided, the light from which is focused, by means of a lens I5, through an aperture IS in an aperture plate I'I, upon the sound record of a moving photographic film I8. The light which passes through this film then impinges ,on the photoelectric cell ii.

The blades of the switch II are connected through a transformer 2i to a potentiometer 22,

which is connected in the input circuit of a space discharge amplifier 24 including tube I24. The output circuit of this amplifier includes the primary of a transformer 25 which is shunted by a resistance 26. The secondary of the transformer -5 25 is connected in the input circuit of a push pull amplifier 21, which comprises two space discharge devices I21. The output circuit of this amplifier includes the primary of a transformer 28, which is shunt ed by a resistance 29. The resistances 26 10 and 29 are provided in order to maintain the frequency characteristics of the circuits independent of the gain ratio characteristics. A meter 30 may be inserted in the plate circuit of the push pull amplifier 21. The secondary of the transformer 15 28 is connected to a high power amplifier 3|, the output of which is connected to a plurality of loud speakers 32, 33.

It is to be understood that additional-amplifiers may be inserted in the photoelectric pick-up 2 circuit as for example between transformer l2 and switch II, if desired.

The potentiometer 22 is shunted by a bank of condensers 23 for controlling the tone quality. The secondary of the transformer 2| is connected 5 through astopping condenser 34 to the input circuit of a space discharge device 39, the output circuit of which is connected to the primary of a transformer 4|, the secondary of which is connected to a circuit including a rectifier 44, 30 fixed resistance 45, and tapped resistance 45. Suitable by-pass condensers 41 and 48 and 49 are provided. A double-pole double-throw switch 52 is used to provide a low initial bias for the amplifiers from an adjustable contact 53 on a 35 resistor assembly 54 when it is not desired to use the dynamic multiplier. In the other position of the switch 52 a bank of condensers 55 is connected across the resistance 46.

The filaments of the devices I24, 39, I21 and 40 44 are supplied with current from the second-. aries of a transformer 56, the primary of which is connected to an A. C. supply 51. An adjustable contact 58 on a resistance 59 is used for providing the initial biasing voltage on the devices 45 I24 and I21.

The cathodespf tubes I21 are joined in parallel and connected to ground, that is the junction of the plate supply battery and the grid bias battery. The cathode of tube I24 is joined 50 to a point somewhat positive with respect to ground as determined by a tap on the plate supply resistor assembly. The grid return circult of device I24 is connected to the junction point of resistors 45 and 46, whereas the grid 55 return circuit of devices |21 is taken from the tap on resistor 46. Thus, in the absence of direct current through resistor 46, the grids are at equal potentials for all three devices, but the bias on tube I24 is slightly in excess of that on tubes I21 because of the difference of cathode connections.

In the operation of the form of the invention shown, with no applied signal, the voltage on the grids of the amplifier 24 and push pull amplifier 21 is determined by the setting of the adjustable contact 58 on resistance '59 which is' so positioned that a suitably small plate current will fiow through the push pull amplifier 21, as indicated by the reading of the meter 30. The

plate current through the amplifier 24 will, for

' termining the relative plate currents of the three devices.

When a signal is received from either the phonograph pick up l0 or the photoelectric cell l3, energy will pass through the transformer 2| to the potentiometer 22 which controls the amount of energy fed to the input circuit of the amplifier 24. The output of this amplifier is passed through the transformer 25 to the input circuit of the push pull amplifier 21. Part of the energy passing through the transformer 2| is applied to the input circuit of the control device 39. The output of this device passes through the transformer 4| and is rectified by the rectifier 44. This rectified current passes through the resistance 46 in the'direction of the arrows,

thus causing a potential difference to be built up across this resistance which is an increasing function of the input signal strength. This potential difference will oppose the biasing voltages of the amplifiers 24 and,21, as determined by the setting of the contact 58, thus decreasing the bias on these devices, which increases the amplification factors of these stages.

A low pass filter consisting of the resistance 45 and the condenser 41 in combination with resistance 46 and condensers 48 and 49 is used to cut out any ripple which might otherwise be presentin the resistance 46.

As the change of bias on the amplifier 24 is determined by the full potential drop across the resistance 46, it will be greater than the change 6f bias on the push pull amplifier 21 which is determined by the potential drop across only a part of this resistance. For example, the variation of bias for the amplifier 24 may be 12 volts and that for the amplifier 21 may be 8 volts. Due to the fact that the initial bias on the amplifier 24 is higher than that on the amplifier 21, the bias voltage on the amplifier 24 may'vary from 20 volts to 8 volts, a range of 12 Volta-while that on the amplifier 21 mayvary from 18 volts to 10 volts, a range of 8 volts. It is thus seen that the bias on the amplifier, 24 will start at a higher value and end at a lower value than the bias on the amplifier 21 and will have a greater range. ,The reason for this difference in bias range is that the second amplifier receives a greater input signal strength than the first and therefore its amplification range can not be so great without producing tonal distortion.

As a result the dynamic range of the amplifier stage including device |24 is greater than the dynamic range of the amplifier stage including device |21. For example, in arranging for total expansion of 24 decibels, an overall change of gain ratio from 10 to 1 is required. The present invention distributes the change of gain ratio between the stages in accordance with theinsignal carrying capacity as for example 5.33 to 1 in the first stage and 3 to 1 in the second stage.

When the system is to be used chiefly for speech, the switch 52 is left open, and when used for music this switch is thrown down, thus throwing the condensers across the resistance 46. By selecting the proper condenser in the bank 55 it is possible to vary the rate of operation of the dynamic multiplier. For example, by increasing the capacity across the resistance 46 the action of the dynamic multiplier is retarded.

If it is desired to eliminate the dynamic multiplier entirely, the switch 52 is thrown up, thus applying a suitable negative voltage from the adjustable contact 53 to the grid returns of the amplifiers 24 and 21 which then operate as normal amplifiers.

The output of the push pull amplifier 21 passes through the transformer. 28 to the input circuit of the high power amplifier 3|, there being amplified and fed to the loud speakers 32 and 33, where it is reproduced as speech or music in the usual manner.

While certain novel features of theinvention have been disclosed and are pointed out in the annexed claims, it will be understood that various omissions, substitutions and changes may be made by those skilled in the art without departing from the spirit of the invention.

What is claimed is:

1. A system for reproducing sound from a sound record comprising a plurality of stages of amplification connected in cascade, the first of said stages being operated at a lower initial amplification factor than the second of said stages, means for varying the amplification factors of said stages in accordance with the average value of the applied signal in such manner that the volume range of the signal is expanded, the amplification factor of said first stage being varied over wider limits than that of said second stage.

2. A system for reproducing sound from a sound record including a pair of stages of amplification arranged in cascade, a common control circuit for said stages, means for operating said control circuit in accordance with the average value of the input signal means for varying the amplifica tion factors of said stages as an increasing function of said average value, said control circuit causing a greater variation in amplification factor in the first of said stages than in the following stage.

3. A system for reproducing sound from a sound record including a pair of stages of amplification arranged in cascade, means for normally applying a bias to each of said stages for controlling the amplification factors thereof, the initial bias on the first stage being greater than that on the subsequent stage and means for reducing the biases of said stages in accordance with the average value of the input signal, the change of bias in said first stage being greater than the change in bias in said subsequent stage.

4. A system for reproducing sound from a sound record comprising a plurality of stages of amplification, each stage including a space discharge amplifier having a control element, impedances in series with said control elements, and means for passing a current through said impedances proportional to the strength of the input signal and in a direction such that the potential drops in the various impedances oppose the normal biases of said control elements, the relationship being such that an increased signal strength decreases the normal negative biases of said control elements and causes an increase in r the amplification ratio of the space discharge devices, said increase in amplification ratio being difierent for each of said devices.

5. In a system for reproducing sound, a sound record, a pick-up device therefor, a multi-stage amplifier fed by said pick-up device, a sound translating device fed by said amplifier, a control circuit fed by said pick-up device and means in said control circuit for controlling the gain ratios of said stages difierently.

6. In a system for reproducing sound, a sound record, a pick-up device therefor, a plurality of stages of amplification, said pick-up device feeding the first stage, a control circuit including an impedance, means in said control circuit for pass ing a current through said impedance in accordance with the strength of the signal, and means by which difierent parts of said impedance control different stages of amplification.

7. The method of reproducing sound from a sound record using a plurality of stages of amplification which comprises controlling the gain ratio of each stage in accordance with the signal strength and producing unequal control effects on the various stages.

8. A system for reproducing sound from a sound record including a pair of stages of amplification arrangedtin cascade, and means to control the amplification factors of said stages in accordance with the average value of the input signal, said means being adapted to eflect unequal variation in the amplification factors of the different stages.

9. The method of reproducing sound from a sound record using a plurality of stages of amplification which comprises operating the stages at diflerent initial gain ratios and varying the gain ratios of said stages over difierent ranges during reproduction of said record.

10. In a system for reproducing sound from a sound record, a pair of stages of amplification arranged in cascade, both said stages being. controlled by a control channel comprising an impedance, two difierent sections in said impedance, said stages being controlled by said two sections, respectively.

11. In a system for reproducing sound from a soundrecord, a pick-up device, anamplifier fed thereby, said amplifier comprising a plurality of stages of amplification arranged in cascade, a

translating device fed by said amplifier, means i JOHN HAYS HAMMOND. JR. ELLISON S. PURINGTON. 

